From Mark Seifrid's chapter on Romans in The Dictionary of the Use of the Old Testament in the New :

He speaks in absolute terms of "death" and "life" for the one who encounters the law because in this passage he presents the human being solely in encounter with the law. Just as in 6:15-23 he considers human beings in themselves without bringing into view their union with Christ (yet, of course, under the power of the gospel), he here considers human beings in themselves without bringing into view their union with Adam (yet, of course, under the power of sin). Apart from the commandment, Paul was once "alive" insofar as he is viewed as an individual apart from his connection with Adam.